284 REPTILIA CHAP. VII 



same way as that described ou p. 12 (Fig. 1), except that 

 the interdorsal elements are suppressed from the beginning. Ii 

 the remaining three pairs of constituent elements of each verte- 

 bra (the basidorsalia, forming the neural arch ; the interventralia, 

 forming the body or centrum ; and the basiventralia) remain 

 separate, the vertebrae are called tPAimospondylous (Te/xvco, I cut, 

 (TTTovSuXo'i, a vertebra). If the neural arches and the centra are 

 suturally united or are fused with each other, the vertebrae are 

 called stereospondylous {crTepe6<i, solid). In many Amniota 

 the atlas or first vertebra remains in a relatively primitive, 

 embryonic condition, and is temnospondylous but for the usual 

 modification that its centrum becomes attached to that of the 

 second vertebra, and forms the odontoid process of the latter. 

 The composition of gastrocentrous vertebrae (cf. p. 282) is best 

 illustrated by the first and second cervical vertebrae of the 

 Crocodile (Fig. 56, 3, p. 28:?!). 



Concerning geographical distribution, even a cursory study 

 shows that the sub-classes have come into existence at very 

 different geological periods, and have each followed their own 

 lines of dispersal. 



